The Firebird Project announces the release of Firebird V1.5.1. this relese represents a committment by the project to develop and deliver ongoing improvements to this popular open source database engine.

I have used it on several large projects, but have recently moved away from it in favor of Postgresql.
Postgresql is just miles away better, it has temp table support and it does not have the notion of PSQL or DSQL

On firebird there is only certain SQL you can use in procedures, but Postgres does not have these limitations.

Also the firebird project has seemed to slack off lately and some of the developers/tech writers are just downright nasty....JS and HB